1. Field of the Invention
The invention pertains to the field of ultrasonic transducers. More particularly, the invention pertains to wear caps for ultrasonic transducers.
2. Description of Related Art
Wear caps with rigid barrels for low-frequency ultrasonic transducers are standardly used in the ultrasonic test and measurement industry. FIG. 1 shows a prior art ultrasonic transducer design with a generally cylindrical body 10, a front surface 12 on the bottom of the cylindrical body 10, and an electrical connector 14 extending from the side of the body 10. For low-frequency transducers, the purpose of a replaceable wear cap is to serve as a low-cost mechanism to protect the active end of the transducer against damage. The wear cap is then replaced when it becomes worn or damaged. Other such wear caps are typically integrated into the design of the transducer body, including designs currently on the market from Olympus NDT Inc. (Waltham, Mass., US) and GE Energy Services (Atlanta, Ga., US).
As shown in FIG. 2 and FIG. 3, a wear cap includes a protective layer 22 that is affixed to the front surface 12 of a transducer and which protects the front transducer surface 12 against damage. The wear cap may include a rigid barrel 20 that slips around the transducer body 10 to hold the protective layer 22 against the front transducer surface 12. An o-ring 26 mounted in the rigid barrel 20 may be used to grip the transducer body 10 and secure the transducer in the wear cap. The rigid barrel 20 of the wear cap may include a notch 24 to receive the electrical connector 14 of the ultrasonic transducer.
To ensure that the stimulus wave generated by an ultrasonic transducer is conducted to the test material and that reflected waves are conducted back to the transducer, the transducer must make good physical contact with the inside surface of the wear cap and the wear cap outside surface must contact the test surface 30 as shown in FIG. 4. In addition, the transducer should be placed perpendicularly to the surface such that the ultrasonic beam 40 is reflected back to the transducer from the test surface. Any misalignment of the transducer in the wear cap causes the ultrasonic beam 50 to be misdirected, as shown in FIG. 5.
U.S. Pat. No. 3,109,155, entitled “PROTECTIVE CASING FOR AN ULTRASONIC TRANSDUCER” and issued Oct. 29, 1963 to Degen, discloses a protective deflector casing for electro-ultrasonic transducers, particularly a single transducer for detection of passing vehicles.
U.S. Pat. No. 4,211,949, entitled “WEAR PLATE FOR PIEZOELECTRIC ULTRASONIC TRANSDUCER ARRAYS” and issued Jul. 8, 1980 to Brisken et al., discloses a linear transducer array for 90-degree or other wide angle sector scans. The transducer array is covered by a body-contacting wear plate made of a material such as filled silicone rubber or polyurethane epoxy in which the longitudinal sound velocity is equal to or less than that in the body and in which the acoustic impedance for longitudinal sound waves is approximately equal to that of the body. Refraction, if it occurs, enhances the field of view without reducing the transmission of acoustic energy. The wear plate provides mechanical support for a fragile front surface matched array.
U.S. Pat. No. 6,072,312, entitled “ENCAPSULATED TRANSDUCER HAVING A PROTECTIVE SLEEVE” and issued Jun. 6, 2000 to Van Den Berg, discloses an encapsulated transducer. The injection-molded encapsulation is a monolith of cured moldable material ensconcing a sensing element proximate the front end and a portion of an information-transmitting medium emanating from the back end. A component alignment pre-form operatively couples the sensing element with the information-transmitting medium. The component alignment pre-form includes a front ferrule and a rear ferrule bonded thereto and linearly spaced apart along a long axis. The component alignment pre-form further includes an annular recess in which the sensing element or coil is placed so that it is linearly spaced and aligned along the common long axis in which the front and rear ferrules are aligned. A first lead of the coil is electrically connected to the front ferrule and a second lead of the coil is electrically connected to the rear ferrule. A back end of the component alignment pre-form receives a stripped end of the cable such that a center conductor mates with the front ferrule and a coaxial conductor mates with the rear ferrule. The conductors are electrically and mechanically connected to the front and rear ferrules. A protective sleeve is then fitted over the coil thereby forming a sleeved coil and cable assembly. This sleeved coil and cable assembly is encapsulated by an injection molding process which provides the durable encapsulation which bonds with itself and with the sleeved coil and cable assembly. The sleeved coil and cable assembly is symmetrically disposed within the encapsulation and the encapsulation includes an integrally formed protective wall having a uniform thickness along a forwardmost portion of the sensing element.
U.S. Pat. No. 6,800,987, entitled “PROTECTIVE HOUSING FOR ULTRASONIC TRANSDUCER APPARATUS” and issued Oct. 5, 2004 to Toda, discloses a protective cover or grid for an ultrasound transducer. The cover includes a series of vertically-spaced members separated from one another by a pre-determined distance. Each member is of uniform width and arranged in a cylindrical shape. The protection grid includes a cavity for a cylindrical transducer. The protection grid operates as both a physical protection mechanism for protecting the housed cylindrical transducer as well as operating as an impedance matching device.
Such wear caps are offered only for low-frequency transducers, typically 5-MHz or lower, although Olympus currently provides a single offering for a 10-MHz transducer. Low-frequency transducers are tolerant of wear caps, because low-frequency ultrasound passes through the wear cap material easily. The basic design of a wear cap is a layer of material that is held against the transducer front face. In some implementations, an assembly of either metal or plastic is used to hold the wear cap layer rigidly against the transducer front surface. In other implementations, a wear cap may have the shape of a cup formed as a wear cap layer affixed to a rigid barrel, and the transducer slides into the barrel until it makes contact with the wear cap layer.
Wear caps are traditionally not offered for high-frequency transducers for several reasons, namely:
1. High-frequency ultrasound attenuates rapidly in most materials and can be strongly attenuated by passing through a wear cap.
2. High-frequency ultrasound is reflected very strongly at boundaries between materials, so the addition of a wear cap layer may cause ultrasound to be reflected by the wear cap rather than transmitted to the test material through the wear cap. In fact, if there is any airspace between the transducer's front face and the inside of the wear cap, the high-frequency ultrasound is completely reflected back to the transducer. Ultrasonic measurements are therefore not possible if the inside surface of the wear cap and the front surface of the transducer do not make good contact.
3. High-frequency ultrasound is highly directional. In a pulse-echo type of ultrasonic test application, the ultrasonic beam emitted by a high-frequency transducer must reach and be reflected by a test material surface and must then return to the transducer, hence the name pulse-echo. The addition of a wear cap between the high-frequency transducer and the test material surface may misalign the transducer relative to the test material surface, causing the reflected ultrasonic beam to miss the transducer and therefore not be detected by the transducer.
Because of the above performance issues, wear caps are not conventionally offered for use with high-frequency ultrasonic transducers. Pulse/echo layer thickness (PELT) gauges utilize high-frequency ultrasonic contact transducers, and there is a need to protect the face of these transducers against physical damage, and also a need to employ an intermediate material layer between the transducers and the test material surface so as to enable thickness measurements to be performed more readily. Rigid plastic wear caps for use with PELT gauge high-frequency ultrasonic transducers have been on the market since the 1990s, but there are many performance degradation issues related to these rigid wear caps.